Abstract: Using of ecofriendly biofertilizers instead of chemical ones in fish aquacultures represents an important aim to minimize water pollution, improve water quality and enhance fish growth rate. The impact of inoculation of two strains of Azospirillum brasilense and/or Azotobacter chroococcum on bacterial count, chemical characteristics of water, Specific Growth Rate (SGR), Aspartate Aminotranseferase (AST), Alanine Aminotransferase (ALT) and histopathological changes were studied in Oreochromis niloticus aquaculture. Water analysis results revealed that Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), NPP, NO3-N and O-PO4 levels were significantly (p<0.05) increased by treatment with Azotobacter bacteria while Azospirillum treatment gave lower levels. On the other hand mixed or single bacterial treatments increased fish SGR especially treated with Azotobacter (34.62% increase in growth). Although the mixed bacterial treatment improved some water parameters it showed higher ALT and AST levels and sever histopathological lesions in fish than that of single treatment. So the present study introduces the single inoculation of Azotobacter bacteria biofertilizer as a suitable probiotic which may be used in Oreochromis niloticus aquacultures.